In a hybrid car and an electric car that are expected as a last resort for reducing CO2 emission, a lithium ion secondary battery is regarded as a key device. As an electrolyte of the lithium ion secondary battery, lithium hexafluorophosphate having high safety and excellent electrical characteristics is exemplified. A hexafluorophosphate including lithium hexafluorophosphate is manufactured by using phosphorus pentafluoride “PF5” as a starting raw material. Phosphorus pentafluoride is a substance that is used as a fluorinating agent of various chemical reactions in the chemical industry and is gaseous at room temperature.
On the other hand, silver hexafluorophosphate “AgPF6” or potassium hexafluorophosphate “KPF6” as a kind of hexafluorophosphates has been attracting special interest as a counter ion that generates an acid necessary for an initiation and growth reaction in photopolymerization. Also, ammonium hexafluorophosphate “NH4 PF6” is useful as a raw material that is used in the manufacture of a pharmaceutical intermediate. Furthermore, quaternary ammonium salts such as triethylmethylammonium hexafluorophosphate and tetraethylammonium hexafluorophosphate are useful as electrolytes for an electric double layer capacitor that is expected as a high power electricity storage device.
As described above, the hexafluorophosphate is used as an indispensable substance depending on functions required in various fields, and PF5 is a very important substance as a raw material in the manufacture of the hexafluorophosphate. However, when the hexafluorophosphate is manufactured by using PF5, there is a common problem about the manufacturing cost of PF5. Particularly, high purity PF5 used in the manufacture of a high quality hexafluorophosphate that can be used as an electrolyte of a lithium ion secondary battery or the like is remarkably expensive.
The methods of manufacturing PF5 and hexafluorophosphate are described in various documents as exemplified below.
Patent Document 1 discloses a method of manufacturing PF5 by thermally decomposing a raw material. For example, in the case of thermal decomposition of LiPF6 (Scheme 1 shown below), the decomposition slightly occurs around 120° C. and LiPF6 decomposes completely around 200° C. to 250° C. Thereafter, a powder of LiF remains. Furthermore, in other cases, it is difficult to decompose PF5 if the decomposition is not performed at a very high temperature such as about 250° C. in the case of NH4PF6, about 400° C. in the case of NaPF6, and 600° C. to 700° C. in the case of KPF6 and CsPF6. Therefore, expensive heat-resistant manufacturing facilities are required, resulting in increased manufacturing cost. Thus, it is hard to say that the manufacturing method disclosed in Patent Document 1 is industrially reasonable.LiPF6(s)→LiF(s)+PF5(g)  [Chemical Formula 1]
In the reactions of fluorination of phosphorous using ClF3 in a liquid hydrogen fluoride medium (refer to Non-Patent Document 1) and fluorination of phosphorous using a fluorine gas (refer to Patent Document 2 and Non-Patent Document 2), there is a problem that it is very difficult to control the reactions because the reactions proceed explosively. Furthermore, since an expensive fluorine gas is used, as a matter of course, the resultant PF5 is also expensive.
Patent Document 3 describes the reaction of fluorination of phosphoryl trifluoride “POF3” using HF in the presence of sulfur trioxide. However, there is a problem that the reaction is inferior in yield and sulfuric acid is produced, resulting in very severe corrosion in the presence of HF.
Patent Document 4 discloses a method of reacting hexafluorophosphoric acid (HPF6) with a sulfur-based acid under a high pressure. However, similarly to Patent Document 3, there is a problem that sulfuric acid is produced, resulting in very severe corrosion in the presence of HF and, even when fuming sulfuric acid is used, water or fluorosulfuric acid (HSO3F) in the system reacts with PF5 and the resultant PF5 is decomposed into POF3.
Non-Patent Document 3 describes that LiPF6 is manufactured by dissolving lithium chloride in HF and adding phosphorus pentachloride thereto. Also, Patent Document 5 describes that hexafluorophosphate is manufactured by reacting phosphorus pentachloride with a HF gas at a temperature within a range from 60 to 165° C. and adding the resultant PF5 to an anhydrous HF solution of an alkali metal fluoride.
However, in the manufacturing methods disclosed in Non-Patent Document 3 and Patent Document 5, since phosphorus pentachloride is a solid having high hygroscopicity and is inferior in workability, there are problems that handling properties are poor when raw materials are charged in the manufacturing facilities, and that it is difficult to attempt mechanization. Also, when phosphorus halides typified by phosphorus pentachloride are used as raw materials, a large amount of hydrogen halide is produced as by-products, thus requiring long and large-size facilities for treatment of an exhaust gas. Furthermore, moisture contained in phosphorus pentachloride is mixed into the reaction system and a portion of the resultant PF5 reacts with the moisture to form phosphorus oxyfluoride such as POF3 or PO2F as a by-product. As a result, when the hexafluorophosphate is LiPF6, oxyfluorophosphoric acid compounds such as LiPOF4 and LiPO2F2 are produced and contaminate products, resulting in deterioration of productivity of LiPF6. Also, when LiPF6 manufactured by the method is used as an electrolyte of a lithium battery, there arises a problem that the oxyfluorophosphoric acid compounds cause deterioration of characteristics of the battery.
In order to alleviate the problems described above, for example, Patent Document 6 discloses the following manufacturing method. First, PF5 is produced by reacting phosphorus pentachloride with anhydrous HF. Next, phosphorus oxyfluoride is separated by cooling a mixed gas of PF5 and hydrogen chloride to the temperature that is the boiling point of phosphorus oxyfluoride or lower and the boiling point of PF5 or higher, for example, −40° C. to −84° C., followed by the reaction with lithium fluoride dissolved in anhydrous HF. According to this method, a small amount of phosphorus oxyfluoride is separated from the mixed gas of large excess hydrogen chloride and PF5. However, phosphorus oxyfluoride cannot be completely separated and it is very hard to perform the separation operation. Since the boiling point and the solidifying point of POF3 as phosphorus oxyfluoride are close to each other, there are problems such as the possibility of occlusion of the collector. Therefore, this manufacturing method is not suited for industrial applications.
Also, Patent Document 7 discloses a method of manufacturing PF5 using, as a phosphorus raw material, a chloride (phosphorus trichloride or phosphorus pentachloride). However, according to this manufacturing method, the resultant PF5 contains hydrogen chloride mixed therein. Since the boiling point of PF5 is −84.8° C., whereas the boiling point of hydrogen chloride is −84.9° C., it is impossible to separate hydrogen chloride from PF5 by a simple method.
The above-described methods of manufacturing PF5 and hexafluorophosphate have various problems such as poor workability, reactions under severe conditions, use of expensive raw materials, and disposal of by-products. Therefore, the manufacturing cost increases. Particularly, high-quality PF5 is expensive since it is difficult to manufacture. As a result, the hexafluorophosphate manufactured using PF5 as the raw material is also expensive. Therefore, in order to manufacture inexpensive and high-quality hexafluorophosphate, it is important to manufacture inexpensive and high-quality PF5. In other words, it is necessary to suppress an adverse influence exerted by mixing-in of impurities, particularly moisture and to adopt a method capable of utilizing, as a starting raw material, a raw material taking working environment into consideration.